RobertMacy prodded the keyboard

On Fri, 25 Jul 2014 06:53:39 -0700, Baron
wrote:

aeio prodded the keyboard

On 24/07/14 22:08, Baron wrote:
aeio prodded the keyboard

...just started snipping....
A reading that small could just as well be leakage. Are you sure
that should be milliampere and not amps ? ie 5 ma.

Yes - 0.005 mA. That is on the 2mA scale. There is nothing showing
on the higher scales and nothing on the AC scale.

In that case I would be inclined to forget about it. But just in
case check it regularly to make sure it isn't getting worse.


Inside every computer is a line filter to prevent the switching
power supply signals from going back out your AC mains, called
conducted EMI. Inside that filter is a cap between NEUTRAL, HOT, and
chassis GND, called a Y-Cap. That y-cap effectively centertaps your
AC mains, but usually the PC case is connected to earth ground,
almost equal to NEUTRAL, so you never measure any voltage there. If
the GND terminal on the outlet to the PC is floating, well you can
get a pretty healthy zap. I can just start to feel 12Vac so this
60-70Vac hurts a LOT! The size of the y-cap is set to just about
provide a maximum of 2mA.

Now such filtering on an isolated DC supply [if the whole thing is
DC isolated from the PC chassis] can also cause the metal housing's
voltage to meander around.

For what it's worth, 'hospital grade' leakage is less than 100uA and
'direct onnection to patient' hospital grade leakage is less than
10uA ...to simulate what that feels like the next time you're in a
store with fluorescent lighting inside a metal trimmed display
cabinet. Gently slide the back of your knuckles along the metal. You
probably will feel the AC mains voltage. Now, *if* you touch an open
wound to that metal! That's a big oweey, hurts like h---.

The only concern I would have about finding strange voltages where
not expected is that that may be a precursor to a failure mode that
would then supply BIG voltage, as in lethal voltage, if/when
something else happens.

I agree 100%. The PC case should be grounded ! It may pay to check
that the earth is good right back to the panel.

--
Best Regards:
Baron.